1. Field of the Invention
The invention relates to apparatus and methods of testing or measuring the bulk mechanical properties of most liquid, gel, and solid materials.
2. Description of the Prior Art
Probing the bulk mechanical properties of most liquid, gel, and solid materials requires applying some form of deformation, often a shear pulse, and measuring the response of the sample. Devices designed to measure such properties include viscometers, rheometers, texture analyzers, and materials testers. To function properly, the tool surfaces of these devices must remain firmly adhered to the test sample; wall slip at either the deformation-side or detection-side interface interferes with testing. But many important materials, including biological tissues, foods, swollen gels, and other dispersed systems, form depletion layers at the interface. Depletion layers and other mechanisms of wall slip can present a significant barrier to investigating affected materials.
Previous attempts to overcome wall slip can be divided into two broad categories, surface modifications to existing geometries and novel geometries. One important rheometric tool to which surface modifications have been made is the parallel plate tool geometry. In the parallel plate tool geometry, the sample is placed between two parallel, smooth discs. As one disc displaces the sample, the other disc measures the magnitude and phase of the resulting stress. The no-slip boundary condition mentioned above is a crucial assumption in all shear rheometry measurements. Surface modifications have included physically roughening, chemically modifying, and scoring tool surfaces with grooves or cross-hatch patterns. But these modifications are not enough to overcome slip in swollen gels and other samples in which a significant lubricating layer develops. Often pressure (normal force) must be applied to maintain good contact. Application of normal force interferes with accurate measurements because it changes the rheological properties of the sample. It can also destroy delicate network structures. Also, irregular surface features may induce complex flow patterns and ill-defined deformation at the tool surface.
More exotic geometries have also been invented for specific applications, such as the vane and helix geometries. Three issues that hinder wide-spread embrace of these tools follow: 1) they often require large sample volumes. 2) determining true rheological properties such as viscosity, modulus etc. is experimentally difficult if possible at all, and 3) most are not suitable for use with delicate samples such as swollen gels and soft biological samples.
What is needed is some type of apparatus and method of measuring the mechanical properties of materials that overcomes each of the foregoing limitations of the prior art.